The clinical core consists of two Baltimore City Health Department (BCHD) Sexually Transmitted Disease (STD) Clinics and John Hopkins University Division of Infectious Diseases. The BCHD STD clinics are located in high STD morbidity areas and provide comprehensive STD and reproductive health care services to over 35,000 patient visits annually. Gonococcal disease is present in over 25% of patients; chlamydia in over 15%; HIV in 5%. Thirty percent of patients evaluated and treated for STDs will have another episode within one year. These clinics have a proven track record in STD research which has been recently enhanced by the implementation of a computerized data management system which facilitates integration of epidemiological, clinical, and laboratory information. The clinics are closely linked to the JHU Division of Infectious Diseases, where additional data management and clinical laboratory support services are available. The clinics will provide longitudinal access to biological specimens for all projects. The clinics will be instrumental in developing a longitudinal, population-based specimen bank for studies of immunological susceptibility to chlamydia and gonococcal infections.